U.S. patent number 10,502,869 [Application Number 13/879,842] was granted by the patent office on 2019-12-10 for optical element with a porous low refractive index layer having a protection layer.
This patent grant is currently assigned to 3M INNOVATIVE PROPERTIES COMPANY. The grantee listed for this patent is William D. Coggio, William F. Edmonds, Encai Hao, Robert F. Kamrath, Ramesh C. Kumar, Lan H. Liu, Michael L. Steiner, John J. Stradinger, John A. Wheatley. Invention is credited to William D. Coggio, William F. Edmonds, Encai Hao, Robert F. Kamrath, Ramesh C. Kumar, Lan H. Liu, Michael L. Steiner, John J. Stradinger, John A. Wheatley.
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United States Patent |
10,502,869 |
Coggio , et al. |
December 10, 2019 |
Optical element with a porous low refractive index layer having a
protection layer
Abstract
An optical article includes an optical element, a low refractive
index layer disposed on the optical element having an effective
refractive index of 1.3 or less and a polymeric protective layer
disposed on the low refractive index layer. The low refractive
index layer includes a binder, a plurality of metal oxide particles
dispersed in the binder, and a plurality of interconnected voids.
The polymeric protective layer does not increase an effective
refractive index of the optical article by greater than 10%.
Inventors: |
Coggio; William D. (Westford,
MA), Kumar; Ramesh C. (Woodbury, MN), Wheatley; John
A. (Lake Elmo, MN), Steiner; Michael L. (New Richmond,
WI), Edmonds; William F. (Minneapolis, MN), Liu; Lan
H. (Rosemount, MN), Hao; Encai (Woodbury, MN),
Kamrath; Robert F. (Mahtomedi, MN), Stradinger; John J.
(Roseville, MN) |
Applicant: |
Name |
City |
State |
Country |
Type |
Coggio; William D.
Kumar; Ramesh C.
Wheatley; John A.
Steiner; Michael L.
Edmonds; William F.
Liu; Lan H.
Hao; Encai
Kamrath; Robert F.
Stradinger; John J. |
Westford
Woodbury
Lake Elmo
New Richmond
Minneapolis
Rosemount
Woodbury
Mahtomedi
Roseville |
MA
MN
MN
WI
MN
MN
MN
MN
MN |
US
US
US
US
US
US
US
US
US |
|
|
Assignee: |
3M INNOVATIVE PROPERTIES
COMPANY (St. Paul, MN)
|
Family
ID: |
44906421 |
Appl.
No.: |
13/879,842 |
Filed: |
October 20, 2011 |
PCT
Filed: |
October 20, 2011 |
PCT No.: |
PCT/US2011/057006 |
371(c)(1),(2),(4) Date: |
April 17, 2013 |
PCT
Pub. No.: |
WO2012/054680 |
PCT
Pub. Date: |
April 26, 2012 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20130202867 A1 |
Aug 8, 2013 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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61405015 |
Oct 20, 2010 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
G02B
1/14 (20150115); G02B 1/11 (20130101); G02B
1/105 (20130101); Y10T 428/24975 (20150115); G02B
2207/107 (20130101); Y10T 428/24997 (20150401) |
Current International
Class: |
G02B
1/10 (20150101); G02B 1/11 (20150101); G02B
1/14 (20150101); G02B 5/02 (20060101); B32B
7/02 (20190101) |
Field of
Search: |
;428/141,142,143 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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193269 |
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EP |
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EP |
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1445095 |
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Aug 2004 |
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EP |
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H11-281802 |
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Oct 1999 |
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JP |
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2001-100003 |
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JP |
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2005-352121 |
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JP |
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2006-171596 |
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JP |
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2006-297680 |
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JP |
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2008-003243 |
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Jan 2008 |
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JP |
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WO 2008020610 |
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Feb 2008 |
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WO |
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WO 2009-062140 |
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May 2009 |
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WO |
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Other References
Nicholson, John W. "The Chemistry of Polymers, 4th edition." RSC
Publising. 2012. cited by examiner .
Paint Flow and Pigment Dispersion, Patton, T.C., 2.sup.nd Edition,
J. Wiley Interscience, 1978, Chapter 5, p. 126. cited by applicant
.
Modeling Cluster Voids and Pigment Distribution to Predict
Properties and CPVC in Coatings. Part 1: Dry Coating Analysis and
Sudduth, R. D. Pigment and Resin Technology, 2008, 37(6). p. 375.
cited by applicant .
Ridgeway, CJ, Transport in Porous Media, 2006, 63, p. 239. cited by
applicant .
Lee, H.K. et al, The Journal of Imaging Science and Technology,
2005, 49, p. 54. cited by applicant .
R, Peng, Yoon et al, Adv. Materials 2008, vol. 20, p. 1934. cited
by applicant .
Hsu, K Y, Leu J. in 2008 Proceedings on Electronic Packaging
Technology and High Density Packaging (ICEPT-HDP 2008, #4607043).
cited by applicant.
|
Primary Examiner: Johnson; Nancy R
Attorney, Agent or Firm: Heiti; Robert V. Fulton; Lisa
P.
Claims
What is claimed is:
1. An optical article comprising: a first optical element
comprising a polarizing film, a diffusing film, a reflecting film,
a retarder, a light guide or a liquid crystal display panel; and a
second optical element comprising: (a) a low refractive index layer
having an effective refractive index of 1.3 or less disposed on the
first optical element, the low refractive index layer comprising: a
binder; a plurality of particles of fumed silica or fumed alumina
dispersed in the binder; wherein the low refractive index layer
comprises a plurality of interconnected voids and has a thickness
of 1 to 20 micrometers; and (b) a water resistant polymeric
protective layer formed from polymers having an average molecular
weight of greater than 50,000 g/mol disposed on the low refractive
index layer, wherein the polymeric protective layer has a thickness
of 1 to 15 micrometers, said polymers comprising: organomodified
silicones selected from the group consisting of (meth)acrylate
modified silicones, silicone modified (meth)acrylates,
silicone-polyureas, silicone-polyurethane-polyureas, silicone
polyamides, and silicone polyoxamides; or thermoplastic silicone
acrylate copolymers; and wherein the polymeric protective layer
does not increase an effective refractive index of the second
optical element by greater than 10% as compared to a second optical
element having no polymeric protective layer.
2. An optical article according to claim 1, wherein the polymeric
protective layer is formed from polymers having an average
molecular weight of at least 100,000 g/mol.
3. An optical article according to claim 1, wherein the polymeric
protective layer comprises cross-linked polymers.
4. An optical article according to claim 3, wherein the polymeric
protective layer is formed from an aqueous emulsion.
5. An optical article according to claim 1, wherein the polymeric
protective layer comprises thermoplastic polymers having a T.sub.g
of 60 degrees centigrade or greater.
6. An optical article according to claim 1, wherein the polymeric
protective layer is a pressure sensitive adhesive.
7. An optical article according to claim 1, wherein the polymeric
protective layer comprises a plurality of haze generating particles
dispersed in the protective layer.
8. An optical article according to claim 7, wherein the haze
generating particles have an average lateral dimension of 1
micrometer or greater.
9. An optical article according to claim 1, wherein the plurality
of particles comprises fumed silica.
10. An optical article according to claim 1, wherein a weight ratio
of the binder to the plurality of particles is 1:2 or less.
11. An optical article according to claim 1, wherein the optical
article has a haze value of at least 75%.
12. An optical article according to claim 1, further comprising a
second low refractive index layer disposed on the optical element
or polymeric protective layer or between the low refractive index
layer and the polymeric protective layer.
13. An optical article comprising: an optical element; a low
refractive index layer having an effective refractive index of 1.3
or less disposed on the optical element, the low refractive index
layer comprising: a binder; a plurality of fumed silica particles
dispersed in the binder; and wherein the low refractive index layer
comprises a plurality of interconnected voids and has a thickness
of 1 to 20 micrometers; and a polymeric protective layer formed
from polymers having an average molecular weight of greater than
50,000 g/mol said polymers comprising crosslinked organo-modified
silicone selected from the group consisting of (meth)acrylate
modified silicones, silicone modified (meth)acrylates,
silicone-polyureas, silicone-polyurethane-polyureas, silicone
polyamides, and silicone polyoxamides; wherein the polymeric
protective layer has a thickness of 1 to 15 micrometers.
14. An optical article according to claim 13, wherein the polymeric
protective layer comprises polymers having an average molecular
weight of at least 100,000 g/mol.
15. An optical article according to claim 13, wherein the polymeric
protective layer comprises polymers having an average molecular
weight of at least 250,000 g/mol.
16. An optical article according to claim 13, wherein the polymeric
protective layer comprises polymers having an average molecular
weight of at least 500,000 g/mol.
17. An optical article according to claim 13, wherein a weight
ratio of the binder to the plurality of particles is 1:2 or
less.
18. An optical article according to claim 13, wherein the optical
article has a haze value of at least 75%.
Description
BACKGROUND
Articles having a structure of nanometer sized pores or voids can
be useful for several applications based on optical, physical, or
mechanical properties provided by their nanovoided composition. For
example, a nanovoided article includes a polymeric solid network or
matrix that at least partially surrounds pores or voids. The pores
or voids are often filled with gas such as air. The dimensions of
the pores or voids in a nanovoided article can generally be
described as having an average effective diameter that can range
from about 1 nanometer to about 1000 nanometers. The International
Union of Pure and Applied Chemistry (IUPAC) has defined three size
categories of nanoporous materials: micropores with voids less than
2 nm, mesopores with voids between 2 nm and 50 nm, and macropores
with voids greater than 50 nm. Each of the different size
categories can provide unique properties to a nanovoided
article.
Several techniques have been used to create porous or voided
articles, including, for example polymerization-induced phase
separation (PIPS), thermally-induced phase separation (TIPS),
solvent-induced phase separation (SIPS), emulsion polymerization,
and polymerization with foaming/blowing agents. Often, the porous
or voided article produced by these methods requires a washing step
to remove materials such as surfactants, oils, or chemical residues
used to form the structure. The washing step can limit the size
ranges and uniformity of the pores or voids produced. These
techniques are also limited in the types of materials that can be
used.
BRIEF SUMMARY
The present disclosure relates to optical elements that include a
low refractive index layer and a polymeric protective layer. In
particular, the present disclosure relates to optical elements that
include a low refractive index layer and a polymeric protective
layer that does not increase the effective refractive index of the
optical element by more than 10%.
In one illustrative embodiment, an optical article includes an
optical element, a low refractive index layer disposed on the
optical element having an effective refractive index of 1.3 or less
and a polymeric protective layer disposed on the low refractive
index layer. The low refractive index layer includes a binder, a
plurality of metal oxide particles dispersed in the binder, and a
plurality of interconnected voids. The polymeric protective layer
does not increase an effective refractive index of the optical
article by greater than 10%.
In another illustrative embodiment, an optical article includes an
optical element, a low refractive index layer disposed on the
optical element having an effective refractive index of 1.3 or
less, and a polymeric protective layer disposed on the low
refractive index layer. The low refractive index layer includes a
binder, a plurality of metal oxide particles dispersed in the
binder, and a plurality of interconnected voids. The polymeric
protective layer is formed from polymers having an average
molecular weight of at least 50,000 g/mol.
In a further illustrative embodiment, an optical article includes
an optical element, a low refractive index layer disposed on the
optical element having an effective refractive index of 1.3 or
less, and a cross-linked polymeric protective layer. The low
refractive index layer includes a binder, a plurality of metal
oxide particles dispersed in the binder, and a plurality of
interconnected voids.
These and various other features and advantages will be apparent
from a reading of the following detailed description.
BRIEF DESCRIPTION OF THE DRAWINGS
The disclosure may be more completely understood in consideration
of the following detailed description of various embodiments of the
disclosure in connection with the accompanying drawing, in
which:
FIG. 1 is a schematic diagram side elevation view of an
illustrative optical article.
The FIGURE is not necessarily to scale. Like numbers used in the
FIGURES refer to like components.
DETAILED DESCRIPTION
In the following description, reference is made to the accompanying
drawing that forms a part hereof and in which is shown by way of
illustration. It is to be understood that other embodiments are
contemplated and may be made without departing from the scope or
spirit of the present disclosure. The following detailed
description, therefore, is not to be taken in a limiting sense.
Unless otherwise indicated, all numbers expressing feature sizes,
amounts, and physical properties used in the specification and
claims are to be understood as being modified in all instances by
the term "about." Accordingly, unless indicated to the contrary,
the numerical parameters set forth in the foregoing specification
and attached claims are approximations that can vary depending upon
the desired properties sought to be obtained by those skilled in
the art utilizing the teachings disclosed herein.
As used in this specification and the appended claims, the singular
forms "a," "an," and "the" encompass embodiments having plural
referents, unless the content clearly dictates otherwise. As used
in this specification and the appended claims, the term "or" is
generally employed in its sense including "and/or" unless the
content clearly dictates otherwise.
Spatially related terms, including but not limited to, "lower,"
"upper," "beneath," "below," "above," and "on top," if used herein,
are utilized for ease of description to describe spatial
relationships of an element(s) to another. Such spatially related
terms encompass different orientations of the device in use or
operation in addition to the particular orientations depicted in
the figures and described herein. For example, if a cell depicted
in the FIGURES is turned over or flipped over, portions previously
described as below or beneath other elements would then be above
those other elements.
As used herein, when an element, component or layer for example is
described as forming a "coincident interface" with, or being "on"
"connected to," "coupled with" or "in contact with" another
element, component or layer, it can be directly on, directly
connected to, directly coupled with, in direct contact with, or
intervening elements, components or layers may be on, connected,
coupled or in contact with the particular element, component or
layer, for example. When an element, component or layer for example
is referred to as begin "directly on," "directly connected to,"
"directly coupled with," or "directly in contact with" another
element, there are no intervening elements, components or layers
for example.
The present disclosure relates to optical elements that include a
low refractive index layer and a polymeric protective layer. In
particular, the present disclosure relates to optical elements that
include a low refractive index layer and a polymeric protective
layer that does not increase the effective refractive index of the
optical element by more than 10%. The polymeric protective layer is
a stable protective layer that does not substantially degrade the
physical properties of the optical element or the low refractive
index layer upon aging. The polymeric protective layer does not
substantially diffuse into the nanoporous low refractive index
layer. The present disclosure describes integrated optics in film
format that can be useful for display applications. The present
disclosure describes protective layers that improve cohesive
strength of a film construction having a low refractive index
layer. While the present disclosure is not so limited, an
appreciation of various aspects of the disclosure will be gained
through a discussion of the examples provided below.
Some embodiments of the diffuser coatings, articles or
constructions of the present disclosure include one or more low
refractive index layers that include a plurality of voids dispersed
in a binder. The voids have an index of refraction n.sub.v and a
permittivity .epsilon..sub.v, where n.sub.v.sup.2=.epsilon..sub.v,
and the binder has an index of refraction n.sub.b and a
permittivity .epsilon..sub.b, where n.sub.b.sup.2=.epsilon..sub.b.
In general, the interaction of a low refractive index layer with
light, such as light that is incident on, or propagates in, the low
refractive index layer, depends on a number of film or layer
characteristics such as, for example, the film or layer thickness,
the binder index, the void or pore index, the pore shape and size,
the spatial distribution of the pores, and the wavelength of light.
In some embodiments, light that is incident on or propagates within
the low refractive index layer "sees" or "experiences" an effective
permittivity .epsilon..sub.eff and an effective index n.sub.eff,
where n.sub.eff can be expressed in terms of the void index
n.sub.v, the binder index n.sub.b, and the void porosity or volume
fraction "f." In such embodiments, the optical film or low
refractive index layer is sufficiently thick and the voids are
sufficiently small so that light cannot resolve the shape and
features of a single or isolated void. In such embodiments, the
size of at least a majority of the voids, such as at least 60% or
70% or 80% or 90% of the voids, is not greater than about
.lamda./5, or not greater than about .lamda./6, or not greater than
about .lamda./8, or not greater than about .lamda./10, or not
greater than about .lamda./20, where .lamda. is the wavelength of
light. In some embodiments, some of the voids can be sufficiently
small so that their primary optical effect is to reduce the
effective index, while some other voids can reduce the effective
index and scatter light, while still some other voids can be
sufficiently large so that their primary optical effect is to
scatter light.
In some embodiments, the light that is incident on a low refractive
index layer is visible light, meaning that the wavelength of the
light is in the visible range of the electromagnetic spectrum. In
these embodiments, the visible light has a wavelength that is in a
range of from about 380 nm to about 750 nm, or from about 400 nm to
about 700 nm, or from about 420 nm to about 680 nm. In these
embodiments, the low refractive index layer has an effective index
of refraction and includes a plurality of voids if the size of at
least a majority of the voids, such as at least 60% or 70% or 80%
or 90% of the voids, is not greater than about 70 nm, or not
greater than about 60 nm, or not greater than about 50 nm, or not
greater than about 40 nm, or not greater than about 30 nm, or not
greater than about 20 nm, or not greater than about 10 nm.
In some embodiments, the low refractive index layer is sufficiently
thick so that the low refractive index layer has an effective index
that can be expressed in terms of the indices of refraction of the
voids and the binder, and the void or pore volume fraction or
porosity. In such embodiments, the thickness of the low refractive
index layer is not less than about 1 micrometer, or not less than
about 2 micrometers, or in a range from 1 to 20 micrometers.
When the voids in a disclosed low refractive index layer are
sufficiently small and the low refractive index layer is
sufficiently thick, the low refractive index layer has an effective
permittivity .epsilon..sub.eff that can be expressed as:
.epsilon..sub.eff=f.epsilon..sub.v+(1-f).epsilon..sub.b (1)
In these embodiments, the effective index n.sub.eff of the optical
film or low refractive index layer can be expressed as:
n.sub.eff.sup.2=f n.sub.v.sup.2+(1-f)n.sub.b.sup.2 (2)
In some embodiments, such as when the difference between the
indices of refraction of the pores and the binder is sufficiently
small, the effective index of the low refractive index layer can be
approximated by the following expression: n.sub.eff=f
n.sub.v+(1-f)n.sub.b (3)
In these embodiments, the effective index of the low refractive
index layer is the volume weighted average of the indices of
refraction of the voids and the binder. Under ambient conditions,
the voids contain air, and thus the refractive index n.sub.v for
the voids is approximately 1.00. For example, a low refractive
index layer that has a void volume fraction of about 50% and a
binder that has an index of refraction of about 1.5 has an
effective index of about 1.25.
In some embodiments, the effective index of refraction of the low
refractive index layer is not greater than (or is less than) about
1.3, or less than about 1.25, or less than about 1.2, or less than
about 1.15, or less than about 1.1. In some embodiments, the
refractive index is between about 1.14 and about 1.30. In some
embodiments, the low refractive index layer includes a binder, a
plurality of particles, and a plurality of interconnected voids or
a network of interconnected voids. In other embodiments, the low
refractive index layer includes a binder and a plurality of
interconnected voids or a network of interconnected voids.
A plurality of interconnected voids or a network of interconnected
voids can occur in a number of methods. In one process, the
inherent porosity of highly structured, high surface area fumed
metal oxides, such as fumed silica oxides, is exploited in a
mixture of binder to form a composite structure that combines
binder, particles, voids and optionally crosslinkers or other
adjuvant materials. The desirable binder to particle ratio is
dependent upon the type of process used to form the interconnected
voided structure.
While a binder resin is not a prerequisite for the porous fumed
silica structure to form, it is typically desirable to incorporate
some type of polymeric resin or binder in with the metal oxide
network to improve the processing, coating quality, adhesion and
durability of the final construction. Examples of useful binder
resins are those derived from thermosetting, thermoplastic and UV
curable polymers. Examples include polyvinylalcohol, (PVA),
polyvinylbutyral (PVB), polyvinyl pyrrolidone (PVP), polyethylene
vinly acetate copolymers (EVA), cellulose acetate butyrate (CAB)
polyurethanes (PURs), polymethylmethacrylate (PMMA), polyacrylates,
epoxies, silicones and fluoropolymers. The binders could be soluble
in an appropriate solvent such as water, ethyl acetate, acetone,
2-butone, and the like, or they could be used as dispersions or
emulsions. Examples of some commercially available binders useful
in the mixtures are those available from Kuraray-USA, Wacker
Chemical, Dyneon LLC, and Rhom and Haas. Although the binder can be
a polymeric system, it can also be added as a polymerizable
monomeric system, such as a UV, or thermally curable or
crosslinkable system. Examples of such systems would be UV
polymerizable acrylates, methacrylates, multi-functional acrylates,
urethane-acrylates, and mixtures thereof. Some typical examples
would be 1,6 hexane diol diacrylate, trimethylol propane
triacrylate, pentaerythritol triacryalate. Such systems are readily
available from suppliers such as Neo Res (Newark, Del.), Arkema
(Philadelphia, Pa.), or Sartomer (Exton, Pa.). Actinic radiation
such as electron beam (E-beam), gamma and UV radiation are useful
methods to initiate the polymerization of these systems, with many
embodiments utilizing UV active systems. Other useful binder
systems can also be cationically polymerized, such systems are
available as vinyl ethers and epoxides.
The polymeric binders can also be formulated with cross linkers
that can chemically bond with the polymeric binder to form a
crosslinked network. Although the formation of crosslinks is not a
prerequisite for the formation of the porous structure or the low
refractive index optical properties, it is often desirable for
other functional reasons such as to improve the cohesive strength
of the coating, adhesion to the substrate or moisture, or thermal
and solvent resistance. The specific type of crosslinker is
dependent upon the binder used. Typical crosslinkers for polymeric
binders such as PVA would be diisocyanates, titantates such as
TYZOR-LA.TM. (available from DuPont, Wilmington, Del.),
poly(epichlorhydrin)amide adducts such as PolyCup 172, (available
from Hercules, Wilmington, Del.), multi-functional aziridines such
as CX100 (available from Neo-Res, Newark, Del.) and boric acid,
diepoxides, diacids and the like.
The polymeric binders may form a separate phase with the particle
aggregates or may be inter-dispersed between the particle
aggregates in a manner to "bind" the aggregates together into a
structures that connect with the metal oxidize particles through
direct covalent bond formation or molecular interactions such as
ionic, dipole, van Der Waals forces, hydrogen bonding and physical
entanglements with the metal oxides.
Exemplary particles include fumed metal oxides or pyrogenic metal
oxides, such as, for example, a fumed silica or alumina. In some
embodiments, particles that are highly branched or structured may
be used. Such particles prevent efficient packing in the binder
matrix and allow interstitial voids or pores to form. Exemplary
materials include highly branched or structured particles include
Cabo-Sil.TM. fumed silicas or silica dispersions, such as, for
example, those sold under trade designations EH5, TS 520, or
pre-dispersed fumed silica particles such as those available as
Cabo-Sperse.TM. PG 001, PG 002, PG 022, 1020K, 4012K, 1015
(available form Cabot Corporation). Fumed alumina oxides are also
useful structured particles to form a low refractive index system
although silica may be preferred since it has an inherently lower
skeletal refractive index than alumina. Examples of alumina oxide
are available under the trade name Cabo-Sperse, such as, for
example, those sold under the trade designation Cabo-Sperse.TM.
PG003 or Cabot Spec-Al.TM.. In some embodiments, aggregates of
these exemplary fumed metal oxides include a plurality of primary
particles in the range of about 8 nm to about 20 nm and form a
highly branched structure with a wide distribution of sizes ranging
from about 80 nm to greater than 300 nm. In some embodiments, these
aggregates pack randomly in a unit volume of a coating to form a
mesoporous structure with complex bi-continuous network of
channels, tunnels, and pores which entrap air in the network and
thus lower the density and refractive index of the coating. Other
useful porous materials are derived from naturally occurring
inorganic materials such as clays, barium sulfates, alumina,
silicates and the like.
Fumed silica particles can also be treated with a surface treatment
agent. Surface treatment of the metal oxide particles can provide,
for example, improved dispersion in the polymeric binder, altered
surface properties, enhanced particle-binder interactions, and/or
reactivity. In some embodiments, the surface treatment stabilizes
the particles so that the particles are well dispersed in the
binder, resulting in a substantially more homogeneous composition.
The incorporation of surface modified inorganic particles can be
tailored, for example, to enhance covalent bonding of the particles
to the binder, thereby providing a more durable and more
homogeneous polymer/particle network.
The preferred type of treatment agent is determined, in part, by
the chemical nature of the metal oxide surface. Silanes are
preferred for silica and other for siliceous fillers. In the case
of silanes, it may be preferred to react the silanes with the
particle surface before incorporation into the binder. The required
amount of surface modifier is dependent upon several factors such
as, for example, particle size, particle type, modifier molecular
weight, and/or modifier type. The silane modifier can have reactive
groups that form covalent bonds between particles and the binder,
such as, for example, carboxy, alcohol, isocynanate, acryloxy,
epoxy, thiol or amines. Conversely, the silane modifier can have
non-reactive groups, such as, for example, alkyl, alkloxy, phenyl,
phenyloxy, polyethers, or mixtures thereof. Such non-reactive
groups may modify the surface of the coatings to improve, for
example, soil and dirt resistance or to improve static dissipation.
Commercially available examples of a surface modified silica
particle include, for example, Cabo-Sil.TM. TS 720 and TS 530. It
may sometimes be desirable to incorporate a mixture of functional
and non-function groups on the surface of the particles to obtain a
combination of these desirable features.
Representative embodiments of surface treatment agents suitable for
use in the compositions of the present disclosure include, for
example, N-(3-triethoxysilylpropyl) methoxyethoxyethoxyethyl
carbamate, N-(3-triethoxysilylpropyl) methoxyethoxyethoxyethyl
carbamate, 3-(methacryloyloxy)propyltrimethoxysilane,
3-acryloxypropyltrimethoxysilane,
3-(methacryloyloxy)propyltriethoxysilane, 3-(methacryloyloxy)
propylmethyldimethoxysilane, 3-(acryloyloxypropyl)
methyldimethoxysilane, 3-
(methacryloyloxy)propyldimethylethoxysilane, 3-(methacryloyloxy)
propyldimethylethoxysilane, vinyldimethylethoxysilane,
phenyltrimethoxysilane, n-octyltrimethoxysilane,
dodecyltrimethoxysilane, octadecyltrimethoxysilane,
propyltrimethoxysilane, hexyltrimethoxysilane,
vinylmethyldiacetoxysilane, vinylmethyldiethoxysilane,
vinyltriacetoxysilane, vinyltriethoxysilane,
vinyltriisopropoxysilane, vinyltrimethoxysilane,
vinyltriphenoxysilane, vinyltri-t-butoxysilane,
vinyltris-isobutoxysilane, vinyltriisopropenoxysilane,
vinyltris(2-methoxyethoxy)silane, styrylethyltrimethoxysilane,
mercaptopropyltrimethoxysilane, 3-glycidoxypropyltrimethoxysilane,
acrylic acid, methacrylic acid, oleic acid, stearic acid,
dodecanoic acid, 2-[2-(2-methoxyethoxy)ethoxy]acetic acid (MEEAA),
beta-carboxyethylacrylate (BCEA), 2-(2-methoxyethoxy)acetic acid,
methoxyphenyl acetic acid, and mixtures thereof.
Particle volume concentration (PVC) and critical particle volume
concentration (CPVC) can be used to characterize the porosity of
the particle binder system used to make the coating. The terms PVC
and CPVC are well defined terms in the paint and pigment literature
and are further defined in frequently referenced articles and
technical books, such as, for example Paint Flow and Pigment
Dispersion, Patton, T. C., 2.sup.nd Edition, J. Wiley Intersceince,
1978, Chapter 5, p. 126 and Modeling Cluster Voids and Pigment
Distribution to Predict Properties and CPVC in Coatings. Part 1:
Dry Coating Analysis and Sudduth, R. D; Pigment and Resin
Technology, 2008, 37(6). p. 375.
When the volume concentration of the particles is larger than CPVC,
the coating is porous since there is not enough binder to fill all
the gaps between the particles and the interstitial regions of the
coating. The coating then becomes a mixture of binder, particles,
and voids. The volume concentration at which this occurs is related
to particle size and particle structure wetting and/or shape.
Formulations with volume concentrations above CPVC have a volume
deficiency of resin in the mixture that is replaced by air. The
relationship between CPVC, PVC and porosity is:
.times..times..times..times..times..times..times..times..times..times.
##EQU00001##
As used in this discussion of CPVC, the term "pigment" is
equivalent to particles and the term "resin" is equivalent to
binder. In certain binder-particle systems, when the volume
concentration of the particles exceeds a critical value known, as
the CPVC, the mixture becomes porous. Thus the coating becomes
essentially a mixture of binder, particles, and air, because there
is insufficient binder to fill all the gaps between the particles
and the interstitial regions of the coating. When this occurs, the
volume concentration is related to at least one of the pigment
particle size distribution, wetting, and the particle structure or
shape. Materials that provide desired low refractive index
properties have submicron pores derived from particle-binder
mixtures that are highly structured and formulated above their
CPVC. In some embodiments, optical articles have CPVC values that
are not greater than (or are less than) about 60%, or less than
about 50%, or less than about 40%.
As described above, particles that are highly branched or
structured prevent efficient packing in the binder matrix and allow
interstitial voids or pores to form. In contrast, material
combinations which fall below the desired CPVC will not be
sufficiently porous. The BET method (described herein) may be
helpful in determining CPVC and thus porosity of low index
materials because the BET method analyzes pores that are less than
200 nm in diameter, less than 100 nm in diameter, or even less than
10 nm in diameter. As used herein, the term "BET method" refers to
the Braunauer, Emmett, and Teller surface area analysis (See S.
Brunauer, P. H. Emmett and E. Teller, J. Am. Chem. Soc., 1938, 60,
309). The BET method is a well-known, scientifically validated
method used to determine pore size, surface area, and percent
porosity of a solid substance. BET theory relates to the physical
adsorption of gas molecules on a solid surface and serves as the
basis for obtaining physical information about the surface area and
porosity of a solid surface. BET data can assist in the
characterization of materials that meet minimum requirements for
forming a porous structure.
The volume concentration of the particles described by the PVC/CPVC
relationship is also related to the weight concentration of the
particles. It is therefore, possible to establish particle weight
ranges that are above the CPVC. The use of weight ratio or weight
percent is one way to formulate mixtures with the desirable CPVC
values. For the optical constructions of the present disclosure,
weight ratios of binder to particle from 1:1 to 1:8 are desirable.
A weight ratio of 1:1 is the equivalent of about 50 wt % particle,
where as 1:8 is equivalent to about 89 wt % particle. Exemplary
binder to metal oxide particle ratios are less than 1:2 (less than
33% binder), less than 1:3, less than 1:4, less than 1:5, less than
1:6, less than 1:7, less than 1:8, less than 1:9, and less than
1:10 (about 8-10% binder). The upper limit of binder may be
dictated by the desired refractive index. The lower limit of binder
may be dictated by the desired physical properties, for example,
processing or final durability characteristics. Thus the binder to
particle ratio will vary depending on the desired end use and the
desired optical article properties.
In general, the low refractive index layer can have any porosity,
pore size distribution, or void volume fraction that may be
desirable in an application. In some embodiments, the volume
fraction of the plurality of the voids in the low refractive index
layer is not less than about 20%, or not less than about 30%, or
not less than about 40%, or not less than about 50%, or not less
than about 60%, or not less than about 70%, or not less than about
80%.
In some embodiments, portions of the low refractive index layer can
manifest some low index properties, even if the low refractive
index layer has a high optical haze and/or diffuse reflectance. For
example, in such embodiments, the portions of the low refractive
index layer can support optical gain at angles that correspond to
an index that is smaller than the index n.sub.b of the binder.
In some embodiments, some of the particles have reactive groups and
others do not have reactive groups. For example in some
embodiments, about 10% of the particles have reactive groups and
about 90% of the particles do not have reactive groups, or about
15% of the particles have reactive groups and about 85% of the
particles do not have reactive groups, or about 20% of the
particles have reactive groups and about 80% of the particles do
not have reactive groups, or about 25% of the particles have
reactive groups and about 75% of the particles do not have reactive
groups, or about 30% of the particles have reactive groups and
about 60% of the particles do not have reactive groups, or about
35% of the particles have reactive groups and about 65% of the
particles do not have reactive groups, or about 40% of the
particles have reactive groups and about 60% of the particles do
not have reactive groups, or about 45% of the particles have
reactive groups and about 55% of the particles do not have reactive
groups, or about 50% of the particles have reactive groups and
about 50% of the particles do not have reactive groups. In some
embodiments, some of the particles may be functionalized with both
reactive and unreactive groups on the same particle.
The ensemble of particles may include a mixture of sizes, reactive
and non-reactive particles and different types of particles, for
example, organic particles including polymeric particles such as
acrylics, polycarbonates, polystyrenes, silicones and the like; or
inorganic particles such as glasses or ceramics including, for
example, silica and zirconium oxide, and the like.
In some embodiments, the low refractive index layers or material
has a BET porosity that is greater than about 30% (which
corresponds to a surface area of about 50 m.sup.2/g as determined
by the BET method), porosity greater than about 50% (which
corresponds to a surface area of about 65-70 m.sup.2/g as
determined by the BET method), greater than about 60% (which
corresponds to a surface area of about 80-90 m.sup.2/g as
determined by the BET method), and most preferably between about
65% and about 80% (which corresponds to a somewhat higher surface
area of values greater than about 100 m.sup.2/g as determined by
the BET method). In some embodiments, the volume fraction of the
plurality of interconnected voids in the low refractive index layer
is not less than (or is greater than) about 20%, or greater than
about 30%, or greater than about 40%, or greater than about 50%, or
greater than about 60%, or greater than about 70%, or greater than
about 90%. Generally it can be shown higher surface areas indicated
higher percent porosity and thus lower refractive index, however,
the relationship between these parameters is complicated. The
values shown here are only for purposes of guidance and not meant
to exemplify a limiting correlation between these properties. The
BET surface area and percent porosity values will be dictated by
the need to balance the low refractive index and other critical
performance properties such as cohesive strength of the
coating.
The optical constructions of the present disclosure can have any
desired optical haze. In some embodiments, low refractive index
layer has an optical haze that is not less than (or is greater
than) about 20%, or greater than about 30%, or greater than about
40%, or greater than about 50%, or greater than about 60%, or
greater than about 70%, or greater than about 80%, or greater than
about 90%, or greater than about 95%. In some embodiments, the low
index refractive layer has a low optical haze. For example, in some
embodiments, the optical haze of the low index refractive layer is
less than about 20%, less than about 10%, less than about 5%, less
than about 3%, less than about 2%, less than about 1.5%, or less
than about 1%.
In some embodiments, portions of neighboring major surfaces of each
two neighboring layers in optical construction are in physical
contact with each other. For example, portions of neighboring major
surfaces of respective neighboring layers in optical construction
are in physical contact with each other. For example, at least 30%,
or at least 40%, or at least 50%, or at least 60%, or at least 70%,
or at least 80%, or at least 90%, or at least 95% of the two
neighboring major surfaces are in physical contact with each
other.
In some embodiments, portions of neighboring major surfaces (major
surfaces that face each other or are adjacent to each other) of
each two neighboring layers in optical construction are in physical
contact with each other. For example, in some embodiments, there
may be one or more additional layers, not expressly shown in the
FIGURES, disposed between the low refractive index layer and
optical elements. In such embodiments, substantial portions of
neighboring major surfaces of each two neighboring layers in
optical constructions are in physical contact with each other. In
such embodiments, at least 30%, or at least 40%, or at least 50%,
or at least 60%, or at least 70%, or at least 80%, or at least 90%,
or at least 95% of the neighboring major surfaces of each two
neighboring layers in the optical construction are in physical
contact with each other.
There are numerous coating techniques known in the art useful to
make the embodiments of this invention. The more common techniques
are, but not limited to, well known roll-to-roll automated
processes such as knife bar, slot die, slide, curtain, roll and
Grauvre coating techniques. It is also possible to coat these
solutions using non-continuous methods such as inkjet, screen,
offset printing, dip and spray coating techniques. While the exact
coating technique is not critical to obtain the low refractive
index properties some techniques enable multiple layers to be
coated onto the substrate simultaneously, which can improve the
economics of the coating process. The desired final application
will dictate which technique if preferred.
FIG. 1 is a schematic diagram side elevation view of an
illustrative optical article 10. The optical article 10 includes an
optical element 20 with a major surface 22, a low refractive index
layer 30 disposed on the major surface 22 of the optical element 20
having an effective refractive index of 1.3 or less, and a
polymeric protective layer 40 disposed on the low refractive index
layer 30. The low refractive index layer 30 is described above and
includes a binder, a plurality of metal oxide particles dispersed
in the binder, and a plurality of interconnected voids. The
polymeric protective layer 40 is disposed on the low refractive
index layer 30. The polymeric protective layer 40 does not increase
the effective refractive index of the optical article 10 by greater
than 10%.
The polymeric protective layer 40 and the low refractive index
layer 30 can have any useful thickness. In some embodiments the
polymeric protective layer 40 has a thickness in a range from 1 to
20 micrometers or from 1 to 15 micrometers and the low refractive
index layer 30 has a thickness in a range from 1 to 30 micrometers
or from 1 to 20 micrometers. In many embodiments, if the low
refractive index layer 30 is thicker than 10-12 micrometers than
the protective layer thickness can be similar in thickness or
greater.
The optical article 10 can have any useful haze value. In many
embodiments the optical article 10 has a haze value of 20% or
greater, 50% or greater, 70% or greater, 75% or greater, or 90% or
greater. In some embodiments, a second low refractive index layer
(not shown) is disposed on the optical element 20 or polymeric
protective layer 40 or between the low refractive index layer 30
and the polymeric protective layer 40, as desired. The second low
refractive index layer can have the effective refractive index of
1.3 or less, and can be similar to the low refractive index layer
30.
The optical element 20 can be any useful optical element. In many
embodiments the optical element 20 is a polarizing film, a
diffusing film, a reflecting film, a retarder, a light guide or a
liquid crystal display panel. In some embodiments the optical
element 20 is a visible light transparent substrate. In some
embodiments, the optical element 20 can be an absorbing polarizer
or a reflective polarizer. Reflective polarizers include fiber,
multilayer, cholesteric, and wire grid reflective polarizers, for
example. Multilayer reflective polarizers include Brightness
Enhancement Film (BEF) and Dual Brightness Enhancement Film (DBEF)
both commercially available from 3M Company, St. Paul, Minn. In
some embodiments, the optical element 20 can be a light redirecting
film and being diffractive and/or refractive. In some embodiments,
the optical element 20 can be a graphic film, triacetate cellulose,
or an optical adhesive.
In some embodiments, the polymeric protective layer 40 is formed
from polymers having an average molecular weight of at least 50,000
g/mol. In some embodiments, the polymeric protective layer 40 is
formed from polymers having an average molecular weight of at least
100,000 g/mol, or at least 250,000 g/mol, or at least 500,000
g/mol.
In some embodiments, the polymeric protective layer 40 is formed
from cross-linked polymers. The polymeric protective layer can
cross link by any number of chemical reactions, such as a catalyzed
or uncatalyzed self-reactive functional groups such as epoxies,
aziridines, isocyanates, N-methanol amine groups, azalactones or
titanate esters. The crosslinking can occur through ester, amide,
condensation reactions, alcohol dehydration, Diels-Alder and
acid-base reactions or be catalyzed by UV or thermal
polymerization. The crosslinking reaction can occur solely within
the protective layer or can also occur at the interface with the
porous low refactive index layer. The latter is an example of an
interfacial reaction which can increase the interfacial cohesive
strength between the nanoporous layer and the protective layer
In some embodiments, the polymeric protective layer 40 is formed
from multi-functional monomers such as trimethanol propane
triacrylate (SR 351) or dipentaerythrotol tetraacrylates (SR 499,
available from Sartomer of Exton, Pa.).
In some embodiments, the polymeric protective layer 40 is formed
from an aqueous emulsion. Such as, for example, ethylene vinyl
acetate dispersion available under the designated trade name
Vinapass.TM. 192, Vinapass.TM. 400, Vinapass.TM. 420 (Wacker
Chemie, Burghausen, Germany), urethane acrylate dispersions such
NeoCryl XK-98, A614 and Neo Pac R 9699, NeoRes (available from DSM,
Newark, Del.) or acrylate dispersions available under the trade
name Rhoplex, such as Rhoplex HA-12 and Rhoplex TR 407 (Rohm Haas,
Philadelphia, Pa.). Rhoplex HA-12 is a self-crosslinking,
non-ionically stabilized aqueous acrylic emulsion. In some
embodiments, the polymeric protective layer 40 is formed from
thermoplastic polymers having a T.sub.g of 60 degrees centigrade or
greater.
In some embodiments, the polymeric protective layer 40 is a
pressure sensitive adhesive. Such as those available as laminating
PSA from 3M Company under the tradename OCA 8171 or 8172 (3M
Company, St. Paul, Minn.). In many of these embodiments, this
pressure sensitive adhesive layer does not include a tackifier or
is free of a tackifier that tends to be a low molecular weight
material.
In some embodiments, the polymeric protective layer 40 is
organo-modified silicones which will not only maintain a low
refractive index but also can act as oil, water and finger print
resistant coatings. In addition these organo-modified silicones
will provide elastomeric characteristic to the protective coating
to mitigate cracking on drying or handling a coated low refractive
index film. An organo-modified silicone comprises of (meth)acrylate
modified silicone, silicone modified (meth)acrylate,
silicone-polyurea, silicone-polyurethane-polyurea and silicone
polyamides or silicone polyoxamides. These materials can be applied
from a water or water/2-propanol dispersion. Examples of these
materials can be found in U.S. Pat. Nos. 5,032,460; 5,202,190;
5,154,962 assigned to 3M Company.
In some embodiments, the polymeric protective layer 40 includes a
plurality of haze generating particles dispersed in the polymeric
protective layer 40. The haze generating particles can be any
useful particle such as polystyrene particles, for example. The
haze generating particles can have any useful diameter such as 0.5
to 5 micrometers or have an average lateral dimension of 1
micrometer or greater, for example. Typical haze generating
particles such as Soken KSR 3A and SX 350H are available from Soken
Chemical and Engineering Co. Ltd. Sayama-Shi, Saitama-Ken,
Japan.
EXAMPLES
All protective coatings were prepared as described below. The
protective overcoats of this invention were coated on PET substrate
S1. The substrate S1 was prepared by coating a low haze, low
refractive index coating solution on a PET film. This solution was
prepared from a mixture of polyvinyl alcohol (PVA) and fumed silica
oxide Cab-O-Sperse.TM. PG022. The PVA resin, Poval.TM. PVA 235, is
an 88% hydrolyzed polyvinyl alcohol available from Kuraray-USA. In
a typical procedure, 5000 g of Cab-O-Sperse.TM. PG 022 dispersion
(20 wt % solids) was added to a 20 L plastic container equipped
with an air driven laboratory mixer and a heating mantle. The
silica dispersion was gently agitated and warmed to 45-50 degrees
C. When the dispersion had equilibrated in this temperature range,
90 g of a pre-warmed 5 wt % aqueous boric acid solution (available
from Sigma-Aldrich of Milwaukee, Wis., corresponding to 35 g boric
acid or 0.035 g boric acid/g silica) was added to the silica
dispersion and was mixed for about 30 min After this time, 100 g of
a low foaming surfactant (10 wt % Tergitol.TM. Min-Foam 1.times. in
water, available from Dow Chemical Midland, Mich.) was added to the
silica-boric acid mixture followed by the addition of 168 g of
polyvinyl alcohol. The PVA was added as 2315 g of an aqueous 7.2 wt
% solution. Upon addition of the PVA, the mixture became very
viscous and an additional 4350 g of DI water added to reduce the
viscosity and ensure adequate mixing. The mixture was agitated
under mild conditions for an additional 20 minutes. After this
time, the coating solution was transferred to a 30 L, pressure pot
container equipped with an air driven agitator and a vacuum system
to and degassed at approximately 600-700 mm Hg for 30-45 min. After
the mixture was degassed, the solids were checked and the mixture
was found to contain 10.2% solids. The final mixture comprised 1
part PVA resin to 6 parts silica on a dry weight basis (1:6 PVA-Si
ratio, 14.3% PVA by weight).
Coating Process:
The low index coating solution described above was coated on 50
micron (2 mil) Dupont-Teijin 689 primed PET film using an automated
knife over roll coating process to produce the low refractive index
coated PET substrate S1. The knife was 41.9 cm (16.5 in) wide and
the coating solution was supplied to the coating reservoir via a
peristaltic pump. The coating solution was degassed and passed
through a 20 micron nominal filter with a hydrophilic filtration
media available from Meisner Filtration Products of Camarillo,
Calif. The coating solution was delivered warm to the solution bank
while the knife and back-up roll were also heated to 38-42.degree.
C. (100.4 to 107.6.degree. F.) to prevent solution gelling. The
knife coating gap ranged from 101.2 microns (4 mils). The line
speed was 4.57m/min (15 fpm). Films were dried in a two zone
convection oven with the first zone set at 46.1.degree. C.
(115.degree. F.) and the second at 79.4.degree. C. (175.degree.
F.). The dried coating was approximately 7-8 microns thick, as
determined by a digital micrometer. The refractive index was
measured to be 1.164 and the film had transmission-haze-clarity
(T-H-C) values of 92%, 4% and 100% respectively.
Process for Making Protective Overcoats:
The protective layers were coated onto S1 using the small
laboratory scale hand spread coatings method described below. The
substrate S1 was held flat by use of a level 14.times.11 in.
(35.6.times.27.9 cm) vacuum table model 4900 available from
Elcometer Inc. of Rochester Hills, Mich. The coating solution was
spread evenly on PET using a wire round coating rods (Meyer rods)
available from RD Specialties of Webster NY or by use of a knife
bar available from Elcometer Inc (Rochester Hills, Mich.). In a
typical procedure, a standard sheet of white paper (8.5.times.11
in) was placed between the vacuum table and optical film to prevent
coating defects associated with the vacuum table. All coatings were
made using a degassed solution to avoid optical defects such as air
bubble and surface cracks. A 5-8 ml sample of the coating solution
was placed near the top of the film and the coating was made using
either a number 45 or 30 Meyer Rod which provided a coating with a
nominal wet thickness of 114-76.2 microns (4.5 or 3.0 mils)
respectively. When a knife bar coater was used, a 50.8-101.6 micron
(2 to 4 mil) knife bar gap provided a coating with a nominal wet
thickness of 25.4 and 50.8 microns (1 to 2 mils) respectively. The
wet coatings were allowed to air dry at room temp for about 2-3
minutes and were then carefully transferred to a flat glass plate
and placed in a forced air oven at 50.degree. C. to dry completely.
The coatings were covered with an appropriately sized aluminum pan
to reduce drying patterns on the film due to air movement in the
oven.
Refractive Index Measurements:
Refractive index (RI) values were determined by use of the prism
coupling method using the Metricon 2010M Prism Coupler available
from Metricon Corp. of Pennington, N.J. The RI (n) was determined
at 633 nm. Accurate determination of the refractive index of the
higher haze coatings was best determined by measuring the
refractive index in the TM polarization state through the PET side
of the coated film. In this process, the prism and the PET side of
the coatings were coupled and the RI measurement was scanned
between n=1.55 to 1.05. This method results in the detection of two
critical angle transitions; one associated with the PET-prism
interface at n=.about.1.495 and another associated with the PET-low
index coating interface. The Metricon raw data were analyzed to
determine the critical angle of this second transition by use of a
200 point smoothing analysis program of the regions above and below
the inflection point of this second critical angle. Two linear
regions were determined from the smoothed data and the intersection
of these two lines corresponded to the inflection point of the
curve and thus the RI of low refractive index coating.
Water Hold Out Test:
In these tests, a small (approximately 10 cm.sup.2 (4 in.sup.2))
section of the coated film was placed on a flat surface and about
0.5 to 1 ml of deionized (DI) water was placed on the protective
coated surface. The behavior of the water droplet provided a
qualitative evaluation of the integrity of the protective over
coat. If the protective layer provided a barrier to water the water
droplet would bead up or wet the surface but not penetrate the
porous low refractive index layer. If the porous low refractive
index layer was not protected or the over coated surface was
hydrophilic the water would penetrate the surface. A "fail" rating
denotes that within one minute, water was absorbed into the
surface. A "pass" rating denotes the water did not penetrate the
surface after 1 minute. A "pass +" rating denotes the water did not
penetrate the surface even after 5 minutes.
Heat Age Testing:
The thermal stability of the protective overcoats was evaluated by
a heat age test. In these tests, a small (approximately 10 cm.sup.2
(4 in.sup.2)) section of the coated film was placed on an aluminum
pan in a standard laboratory oven at the temperature and times as
indicated in the tables. The refractive index values of the films
were determined as described previously and are reported in the
experimental tables.
Coated Article Optics:
Transmission, haze and clarity values were determined using a
BYK-Gardner Haze Gard Plus (available from BYK-Gardner USA of
Columbia, Md.). The reported values represent the average of at
least 3 measurements taken from different regions of the coated
film. The clarity value calculation uses the ratio (T2-T1)/(T1+T2),
where T1 is the transmitted light that deviates from the normal
direction between 1.6 and 2 degrees, and T2 is the transmitted
light that lies between zero and 0.7 degrees from the normal
direction.
Coated Article Optics:
Transmission, haze and clarity values were determined using a
BYK-Gardner Haze Gard Plus (available from BYK-Gardner USA of
Columbia, Md.). The reported values represent the average of at
least 3 measurements taken from different regions of the coated
film. The clarity value calculation uses the ratio
(T.sub.2-T.sub.1)/(T.sub.1+T.sub.2), where T.sub.1 is the
transmitted light that deviates from the normal direction between
1.6 and 2 degrees, and T.sub.2 is the transmitted light that lies
between zero and 0.7 degrees from the normal direction.
Materials Used to Make Protective Overcoat Solutions:
(CE-1 and EX-1) Ethylene vinyl acetate (Vinnapas.TM. 400 and
Vinnapas.TM. 192) were obtained from Wacker Chemie of GmbH,
Burghausen, Germany. Vinnapas.TM. 400 is described as a
non-crosslinking grade of EVA and Vinnapas.TM. 192 is described as
a self crosslinking grade of EVA. Both materials are provided as
51% solids dispersions in water.
Preparation of Coating Solution CE-1:
100 g of Vinnapas.TM. 400 was weighed into an 800 ml plastic
beaker. 200 g of DI water and 0.5 g of a 10% solution of
Tergitol.TM. Min-Foam 1.times. (Available from Dow Chemical
Midland, Mich.) was added to the EVA dispersions. The components
were mixed thoroughly at low shear using an air driven laboratory
mixer for about 5 minutes. The mixture was transferred to a 500 ml,
1-neck round bottom flask and placed on a rotary evaporator system
(available as a Rotovaptm from Buchi GmbH Flawil Switzerland) at
40.degree. C. and 600 mm Hg vacuum to degassed the mixture. The
coating solution was found to contain approximately 17% solids and
was used to make the protective overcoat layer CE-1. The solution
CE-1 was coated using a knife bar with a 50.8 micron (2 mil)
gap.
Example EX-1:
Was prepared in essentially the same manner as CE-1 but the
dispersion used was Vinnapas.TM. 192.
Preparation of Coating Solution CE-2:
Was prepared in essentially the same manner as CE-1 but 33 g of
Soken KSR 3A crosslinked polystyrene beads were added to the
mixture. (KSR 3A was obtained from Soken Chemical and Engineering
Co. Ltd. Sayama-Shi, Saitama-Ken Japan.) The addition of the beads
increased the solids and the viscosity of the mixture. Therefore,
an additional 150 g of DI water was added to the mixture. This
mixture was agitated further until the beads were well dispersed.
The final mixture was degassed as described for CE-1 and was
approximately 18% solids. CE-2 contained on a dry solids basis 40%
wt KSR3A beads and 60 wt % Vinnapas.TM. 400.
Example EX-2:
Was prepared in essentially the same manner as CE-2 but the
dispersion used was Vinnapas.TM. 192.
Coatings Solutions CE-1 and 2 and EX 1 and 2 were over coated on
Substrate S1 as described above with a knife bar gap of 101.2
microns (4 mil) to produce an approximate wet coating thickness of
50.8 microns (2 mil). The impact of heat aging on Refractive Index
is shown in Table #1.
TABLE-US-00001 TABLE #1 Impact of heat aging on the RI of ULI
layers with Protective Over Coats 90.degree. C. Heat aging
Refractive Index After Heat Aging Sample Over Coat Type 0 Hrs 37
hrs 185 hrs CE-1 EVA 400 only 1.182 1.243 1.222 EX-1 EVA 192 Only
1.168 1.191 1.189 CE-2 EVA-400 + KSR 1.167 1.217 1.239 3A Beads
EX-2 EVA-192 + KSR 1.168 1.17 1.174 3A Beads
The data in Table 1 show that the refractive index change after
heating for the protective layer derived from for the
self-crosslinking EVA 192 is less than the refractive index change
for the non-crosslinking EVA 400.
Coating solutions CE-3 and EX-3-5 were prepared from
Polymethylmethacrylate resins obtained from Sigma Aldrich Chemicals
of Milwaukee, Wish. The PMMA coating solutions were prepared at 10%
solids by dissolving the PMMA in 2-butanone at room temperature.
The mixtures were agitated slowly on a standard laboratory
oscillating mixer for 16 hours to ensure complete dissolution of
the polymers. Once the polymers were completely dissolved the
coating solutions they were allowed to sit at room temperature for
at least one hour to allow any dissolved air to dissipate from the
solutions. Sample CE-3 was prepared using 15,000 g/mol PMMA; EX 3
was prepared from 100,000 g/mol PMMA; EX-4 was prepared from
320,000 g/mol PMMA, EX-5 was prepared from 990,000 g/mol PMMA. The
impact on the RI of the coating layers is shown in Table #2.
TABLE-US-00002 TABLE #2 Impact of polymer molecular weight used for
protective overcoats on the RI of the low index layer PMMA over
coats on Low RI Layer S1: Impact of Molecular weight on RI 320K g/
990K g/ No PMMA 15K g/mol 100K g/mol mol mol Sample S1 CE-3 EX-3
EX-4 EX-5 Initial RI on 1.168 1.315 1.212 1.222 1.224 S1 Water Hold
Fail Pass Pass Pass Pass drop test
The data in Table 2 shows the lower molecular weight PMMA is able
to penetrate the pores compared to the higher molecular weight
protective over coats.
In a manner similar to the preparation used for CE-2, the coating
solutions used for EX-3 (100,000 g/mol PMMA) and EX-5 (990,000
g/mol PMMA) were modified by the addition of 40 wt % Soken KSR 3A
polystyrene diffuser beads to make a protective diffuser solutions
EX-6 and EX-7 respectively. In these examples the solvent used was
MEK and instead of water as was used for CE-2. These compositions
form a protective diffuser layer that modifies the optical
properties but also provides protection of the low refractive index
porous layer. The optical data of these diffuse protective
overcoats are summarized in Table #3.
TABLE-US-00003 TABLE #3 PMMA Diffuser over coats on Low RI Layer
S1: Impact of heat aging on RI RI After 185 hrs Sample Description
RI-initial at 90 C. T-H-C S1 No over coat 1.164 1.164 92-4-100 Ex-6
PMMA-100k 1.214 1.22 78-98-12 g/mol + 40% KSR 3A EX-7 PMMA-990k
1.20 1.190 88-97-6 g/mol + 40% KSR 3A
Another category of materials that can be advantageously used as
protective overcoats are organo-modified silicones. The
organo-modified silicone comprises of (meth)acrylate modified
silicone, silicone modified (meth)acrylate, silicone-polyurea,
silicone-polyurethane-polyurea and silicone polyamides or silicone
polyoxamides. The following (meth)acrylate modified silicone
samples were use to prepare prepared protective overcoats useful in
this invention.
EX-8 Preparation of Silicone Acrylate Protective Overcoat
Solution:
An amber-colored quart bottle was charged with 50 g methyl acrylate
(MA), 20 g methyl methacrylate (MMA), 5 g methacrylic acid (MAA),
25 g mercapto functional silicone (KF-2001), available from
Shin-Etsu, 0.25 g azobisisobutyronitrile (AIBN) initiator and 150 g
methyl ethyl ketone (MEK) solvent. The sealed bottle containing the
solution was tumbled in a constant temperature bath at 55.degree.
C. for 48 hr. The resulting polymer solution at 39.5% solids was
inverted to water by adding 570 g of DI H2O and 3.7 g ammonium
hydroxide. The mixture was put on a table top shaker for about 30
minutes to obtain a homogenous dispersion. The resulting dispersion
was subjected to a vacuum strip at about 45.degree. C. to strip-off
MEK. The final slightly milky looking dispersion was obtained at
about 15% solids in H2O.
Coating solution EX-8 was prepared from this polymer solution by
diluting 35 g of the solution (containing 5.25 g solids polymer) to
100 g using 65 g of solvent blend. The blend comprised 77 wt %
2-propanol (IPA) and 13 wt % DI water. This produced the final
coating solution EX-8 at 5% solids in a 50/50 IPA-Water solvent
blend. The substrate S1 was coated with EX8 solution using a knife
bar with a 101.6 micron (4 mil) bar gap and dried as described
previously.
EX-9 Preparation of Silicone Acrylate Protective Overcoat
Solution:
This polymer solution was prepared in a manner similar to EX-8
except the composition of this polymer had an increased amount of
MMA at the expense of the MA to increase the Tg of the polymer. An
amber-colored quart bottle was charged with 18.2 g methyl acrylate
(MA), 45.4 g methyl methacrylate (MMA), 9.1 g methacrylic acid
(MAA), 27.3 g mercapto functional silicone (KF-2001), available
from Shin-Etsu, 0.25 g azobisisobutyronitrile (AIBN) initiator and
150 g methyl ethyl ketone (MEK) solvent. The sealed bottle
containing the solution was tumbled in a constant temperature bath
at 55 C for 48 hr. The resulting polymer solution at 39.2% solids
was inverted to water by adding 900 g of DI H2O and 6.5 g ammonium
hydroxide. The mixture was put on a table top shaker for about 30
minutes to obtain a homogenous dispersion. The resulting dispersion
was subjected to a vacuum strip at about 45.degree. C. to strip-off
MEK. The final slightly milky looking dispersion was obtained at
about 10% solids in H2O. Coating solution EX-9 was prepared from
this polymer solution by diluting 50 g of the solution (containing
5 g solids polymer) to 100 g using 50 g of a solvent blend. The
blend comprised 77 wt % 2-propanol (IPA) and 13 wt % DI water. This
produced the final coating solution EX-9 at 5% solids in a 38/62
IPA-Water solvent blend. The substrate S1 was coated with EX-9
solution using a knife bar with a 101.6 micron (4 mil) bar gap and
dried as described previously.
TABLE-US-00004 TABLE #4 Silicone acrylate over coats on Low RI
Layer S1: Impact of heat aging on RI water hold out test.
90.degree. C. Heat aging Refractive Index After Heat Aging Water
Sample Over Coat Type 0 Hrs 24 hrs hold out EX-8 Silicone Acrylate
1.164 1.185 Pass+ 20 wt % MMA Ex-9 Silicone Acrylate 1.161 1.168
Pass+ 45 wt % MMA
The data in Table #4 is shows the silicone acrylate can form
protective over coats that do not penetrate the pore and increase
the refractive index and that the overcoats have improved water
resistance as measured by the water hold out test.
Ex 10-12:
Protective over coats prepared from urethane and urethane acrylate
dispersions were from pared from NeoCryl XR-98 (EX-10), NeoCryl 614
(EX-11), NeoPac R-9699 (EX-12). The resins are available as
.about.37% aqueous dispersions from DMS-NeoRes of Wilmington, Mass.
The mixtures were diluted to 22% wt solids with water. The
solutions were coated on S1 using a #12 wire-wound rod (obtained
from RD Specialties, Webster, N.Y.) to form approximately a 1.2 mil
wet layer of the coating solution. The coatings were then dried in
an oven at 65.degree. C. for 2 minutes, The refractive indices of
the films were measured as summarized below in Table #5. RI refers
to refractive index, and T-H-C refers to transmission, haze and
clarity respectively.
TABLE-US-00005 TABLE 5 Performance evaluation of the resulting
protected low index elements RI After 24rs Water Sample Description
RI-initial at 85.degree. C. hold out S1 No over coat 1.164 1.161
Fail EX-10 NeoCryl XK-98 1.170 1.171 Pass+ EX-11 NeoCryl A-614
1.164 1.167 Pass+ EX-12 Neo Pac R 9699 1.161 1.161 Pass+
The data in Table #5 show the self crosslinking urethane acrylate
can form protective over coats that do not penetrate the pore and
increase the refractive index and that the overcoats have improved
water resistance as measured by the water hold out test
Thus, embodiments of the PROTECTED LOW REFRACTIVE INDEX OPTICAL
ELEMENT are disclosed. The implementations described above and
other implementations are within the scope of the following claims.
One skilled in the art will appreciate that the present disclosure
can be practiced with embodiments other than those disclosed. The
disclosed embodiments are presented for purposes of illustration
and not limitation, and the present disclosure is limited only by
the claims that follow.
* * * * *